Fire safety systems include, among other things, detection devices and notification devices. Detection devices include smoke, heat or gas detectors that identify a potentially unsafe condition in a building or other facility. Detection devices can also include manually operated pull stations. Notification devices, often referred to as notification appliances, include horns, strobes, and other devices that provide an audible and/or visible notification of an unsafe condition, such as a “fire alarm”.
In its simplest form, a fire safety system may be a residential “smoke alarm” that detects the presence of smoke and provides an audible alarm responsive to the detection of smoke. Such a smoke alarm device serves as both a detection device and a notification appliance.
In commercial, industrial, and multiple-unit residential buildings, fire safety systems are more sophisticated. In general, a commercial fire safety system will include one or more fire control panels that serve as distributed control elements. Each fire control panel may be connected to a plurality of distributed detection devices and/or a plurality of distributed notification appliances. The fire control panel serves as a focal point for problem-indicating signals that are generated by the distributed detection devices, as well as a source of activation (i.e. notification) signals for the distributed notification appliances. Most fire safety systems in larger buildings include multiple fire control panels connected by a data network. The fire control panels employ this network to distribute information regarding alarms and maintenance amongst each other. In such a way, notification of a fire or other emergency may be propagated throughout a large facility.
Moreover, centralized control of multiple fire control panels in large safety systems can be accomplished by a dedicated or multi-purpose computing device, such as a personal computer. Such a centralized computing device, sometimes referred to as a control station, is typically configured to communicate with the multiple fire control panels via the data network.
Using this general architecture, fire safety systems are scalable to accommodate a number of design factors, including the building layout, the needs of the building management organization, and the needs of the users of the building. To achieve scalability and flexibility, fire safety systems may include, in addition to one or more control stations, remote access devices, database management systems, multiple networks of control panels, and literally hundreds of detection and notification devices. Fire safety systems may further incorporate and/or interact with security systems, elevator control systems, sprinkler systems, and heating, ventilation and air conditioning (“HVAC”) systems.
One of the many sources of costs in fire safety systems is the wiring and material costs associated with the notification appliances. Building safety codes define the specification for notification appliance wiring, voltage and current. For example, according to building safety codes, notification appliances are intended to operate from a nominal 24 volt signal which provides the power for the notification appliance to perform its notification function. For example, an alarm bell, a strobe light, or an electronic audible alarm device operates from a nominal 24 volt supply. In general, however, notification devices are required to operate at voltages as low as 16 volts. The delivery of power to the distributed notification appliances requires a significant amount of wiring and/or a significant number of distributed power sources.
In particular, notification appliances are typically connected in parallel in what is known as a notification appliance circuit or NAC. Each NAC is connected to a power source, such as a 24 volt source, and includes a positive conductor, a ground conductor, and multiple notification appliances connected across the two conductors. The power source may be disposed in a fire control panel or other panel. The positive and ground NAC conductors serve to deliver the operating voltage from the 24 volt power source, to the distributed notification appliances. Because the positive and ground conductors have a finite conductance, i.e. they have impedance, there is a practical limit to how long an NAC may extend from the power source before the voltage available across the NAC conductors falls below the required operating voltage.
To address the limitations of NACs due to voltage drop, extending the coverage of notification appliances often requires increasing the number of power sources. To this end, special powered appliance circuit extension devices may be employed. These powered extension devices are panels that are connected to an existing fire control panel and emulate a notification appliance or device to that fire control panel. Each powered extension device then provides NAC powered signals to additional NACs. The power extension device thus forms a form of “repeater” for the notification signal voltage. The use of the powered extension devices effectively extends the coverage beyond that may be achieved with a single fire control panel. The powered extension device is less costly to implement than a fire control panel.
To date, one of the issues relating to the powered extension devices includes the reliability of the switching elements used to connect alarm signals to the NAC. Switching elements are necessary to controllably connect the 24 volt alarm notification signal to the NAC. In particular, in the past, when an extension device would receive an “alarm notification signal” from its corresponding fire control panel, the extension device would connect its own 24 volt power supply to its extended NAC using a relay. Relay contacts, however, present undesirable reliability issues. While some reliability issues may be partly addressed by using high quality relays, such relays significantly increase the cost of implementation.
Accordingly, there exists a need to reduce costs and increase reliability in notification appliance circuits of fire safety systems, as well as the devices that provide power to those notification appliance circuits.